1. Field of the Invention
The present invention relates to a thickness detecting sensor for detecting the thickness of a medium of a thin paper sheet shape such as a sheet of paper for a copy, an OHP film, a banknote, etc., and more concretely relates to a thickness detecting sensor able to detect the thickness of the medium correspondingly to electrostatic capacity stored between electrodes by changing the gap between these electrodes correspondingly to the thickness of the medium.
2. Description of the Related Art
The thickness detecting sensor for detecting the thickness of the medium is arranged in a copying machine, etc. in recent years. This thickness detecting sensor generally detects whether paper sheets sequentially sent out of a cartridge, etc. are overlapped or not. When the thickness detecting sensor detects a predetermined thickness or more, the thickness detecting sensor judges that it is the overlap sending, and stops the conveyance, etc. With respect to a sensor for detecting a contact state of the medium by coming in contact with this medium, a sensor disclosed in patent literature 1 is known.
[Patent Literature 1]
JP-A-5-288619
This sensor is constructed by arranging a fixing electrode arranged in a base film and a movable electrode arranged so as to be opposed to this fixing electrode. The movable electrode is displaced onto the fixing electrode side by making the medium come in contact with a member arranged on the movable electrode side. The contact state with the medium is detected on the basis of a change in electrostatic capacity caused by changing the gap between these electrodes.
When the electrostatic capacity stored between the electrodes is detected, this electrostatic capacity is converted into a frequency signal by a CR oscillating circuit, etc., and this frequency is detected. However, with respect to the relation of this frequency and the gap between these electrodes, it is known that the relation as shown in FIG. 10 is formed. Namely, in a small area of the gap between the electrodes, the frequency is linearly changed correspondingly to an increase in the gap. In a large area of the gap between the electrodes, the frequency is only gradually changed even when the gap is increased. Therefore, when the gap is widely set in advance, there is a possibility that no frequency is linearly changed even when the gap is changed. Therefore, it is difficult to exactly detect the change in frequency. On the other hand, when the gap is narrowly set in advance, the frequency is linearly changed correspondingly to the change in the gap. Therefore, it is possible to exactly detect the change in frequency. However, when the gap is narrowly set in this way, the movable electrode can be displaced on the fixing electrode side only within the gap range. Therefore, when the medium having a thickness exceeding this gap range comes in contact with the sensor, this sensor is broken. Further, a dynamic range of the sensor is limited to the range of the gap between the electrodes so that the dynamic range is also reduced.